Granulocytes and monocytes are derived from a small population of bone marrow progenitor cells that can be assayed in vitro by their ability to form colonies of myeloid cells in semi-solid medium (colony-forming unit, CFU-C). This colony formation requires the presence of specific glycoproteins termed colony stimulating factors (CSF), produced by activated T cells and monocytes. CFU-C are present in low frequency in bone marrow, and it has been very difficult to isolate these cells. As a result, very little is known about these critical hematopoietic stem cells, particularly about the events that regulate their proliferation and differentiation at the cellular level. The major objective of these studies is the development of techniques to improve our ability to study CFU-C. Monoclonal antibodies are ideal reagents to identify and manipulate such small populations of cells, but it has been previously difficult to generate antibodies reactive with CFU-C surface antigens because purified CFU-C have not been available for immunization. Using a series of existing monoclonal antibodies, we have developed a method to produce large numbers of highly purified CFU-C from the peripheral blood of chronic myeloid leukemia patients. These purified CFU-C will be used for immunization and screening of new monoclonal antibodies. A rapid assay (24 to 48 hrs) of CSF-dependent CFU-C proliferation has been developed that will greatly facilitate the screening procedure. This new assay system can also be modified to identify antibodies that block receptors for CSF. The production of antibodies reactive with CFU-C will potentially allow us to ask a number of important questions about hematopoietic cells. These monoclonal antibodies will be used to identify and purify subpopulations of CFU-C and to investigate the relationships of these cells with progenitor cells of other cell lineages. Antibodies that react with the cell surface receptors for CSF could potentially be the most valuable in this regard. Finally, although these studies primarily involve normal hematopoietic cells, the new monoclonal antibodies may ultimately facilitate study of several human diseases in which myeloid differentiation is abnormal, including acute myeloblastic leukemia, myelodysplastic syndromes, and myeloproliferative disorders. (CS)